Branch guide coupler



E. M. PURCELL BRANCH GUIDE COUPLER Filed Jan. '7, 1946 June 26, 1951 FIG. I

INVENTOR! EDWARD M. PURCELL BY 94a ATTORNEY Patented June 26,1951

UNITED STATES PATENT OFFICE BRANCH GUIDE COUPLER Edward M. Purcell, Cambridge, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application January 7, 1946, Serial No. 639,641

5 Claims. 1

This invention relates to an electrical apparatus and more particularly to a microwave energy coupler.

The microwave coupler here disclosed falls in the general class of directional couplers in which a length of wave guide (main guide) included in a transmission line, and another length of wave guide (secondary guide) connected to a utilization circuit, have several paths of energy transfer between them to achieve directionality or Wave selectivity in the coupling action.

Referring first to certain prior art directional couplers of this class, each energy transfer path includes a relatively small circular aperture, in an enclosing wall of the main guide, as a coupling means between the main and secondary guides. It is a characteristic of such couplers that the coupling of wave energy from the main guide to the secondary guide is accompanied by a certain amount of reaction upon wave conditions in the main guide. Attempts to increase the coupled power beyond approximately one per cent by utilizing apertures of greater size result in unduly large disturbances in the main guide.

It is an object of the present invention to provide a coupler which produces a relatively small disturbance in the transmission line from which it extracts power.

It is another object of the invention to provide a coupler in which the fraction of microwave power extracted or diverted from a transmission line in which the coupler is connected may be relatively large as compared to that extractable by couplers generally.

Another object of the invention is to provide a coupler in which the proportion of microwave power extracted from a transmission line is substantially constant over an operating frequency band.

These and other objects and advantages of the invention will be apparent from the following description when read in connection with the drawing, in which like parts are designated by like reference numerals. In the drawing:

Fig. 1 is a schematic isometric view of the basic structure of the present invention, in part cut away to indicate more clearly the passages through which energy transfer takes place;

Fig. 2 is a side elevation of a practical embodiment of the disclosed invention;

Fig. 3 is an end view of Fig. 2 taken on line 3-3; and,

Fig. 4 is an isometric view of an adapter by means of which two lengths of wave guide may be associated in accordance with the invention.

The invention is illustrated in Fig. l, in which certain walls of the structure are in part cut away to expose internal details. A main guide 5, forming part of a microwave transmission line, is structurally connected and electromagnetically coupled to a secondary guide 6 by a pair of branch guides l and 8. Each wave guide may be rectangular in cross-section as shown, and dimensioned to operate in the dominant (TEo,1) mode in which the electric field vectors of the energy involved extend perpendicularly to the broader walls. Branch guides l and 8 are relatively short and make T-junctions with the wider walls of the main and secondary guides, the longitudinal axes of the component guides being coplanar. The main and secondary guides are alike in cross section, the larger and smaller crosssectional dimensions here bein designated a and b, respectively. The larger cross-sectional dimension of the branch guides is the same as that of the main and secondary guides, but the narrow cross-sectional dimension 1) of the branch guides is smaller than the corresponding dimension 1) of the main and secondary guides. The transverse slots formed at the junctions in the main and secondary guides each have the length a and width b and provide unrestricted passage of energy through the branch guides.

In terms of the relative directions of the E-fields (electric fields) within the guides, each junction formed in the illustrated connection of wave guides may be designated as an E-plane T. The impedances of the guides forming such a T are effectively in series, and for this reason the interconnectin branch guides may be termed seriesbranching guides.

The characteristics of the directional coupler here disclosed are largely dependent upon the spacing of the series branching guides I and 8 along the main and secondary guides 5 and 6, upon the ratio of b and b, and upon the lengths of the branch guides. In terms of effective electrical distances, the branch guide lengths and the spacing of the transverse slots are odd multiples of a quarter guide-wavelength, and, in the embodiment here described, are each a quarter guide-wavelength. Under this condition, the series loading of the main guide by the branch and secondary guides is essentially resistive, and, for given dimensions b and 1), maximum coupling is secured with minimum reflection.

One of the important operating characteristics of the branch guide coupling between main and secondary guides is that, using main and secondary guides having cross-sectional dimensions viously mentioned is limited to relatively small percentages by the nature of circular aperture coupling, and by the attendant relatively large standing Wave ratios. The voltage standing wave ratio, looking into the main guide of such aperture-coupled guides, is very nearly equal to 1+\/a, where a is the fractionof power in the main guide which is diverted to the secondary guide. Thus, if as large an amount as 1.0% of the power is removed, a voltage standing wave ratio of 1.1 is set up. The diversion of an equal percentage of power from a transmission line by use of the present invention sets up a much smaller standing wave ratio, approximately equal to 1+a. For example, if 1.0% of power is removed by the improved coupler here disclosed, a voltage standing wave ratio having the very small value of 1.01 is set up.

An estimate of the coupling ratio (relative amount of energy transferred from the main guide to the secondary guide) is given by the square of the ratio of b to b, and this estimate is a close approximation when b is relatively small as is usual in couplers. Thus, if b is onetenth as large as b, the coupling between the main and secondary guides is substantially one percent. Similarly, if b is 0.224 as large as b, the coupling is approximately five percent.

The actual physical dimensions of branch guide length and spacing for optimum performance are dependent upon the cross-sectional dimensions of the main and secondary guides, and upon the mid-frequency value of the desired operating range. The disclosed structure is amenable to an exact theoretical analysis in this respect, for each T-junction can be replaced by corresponding parameters in equivalent transmission line circuits.

The effective quarter guide-wavelength dimen sions of the length and spacing of the branch guides achieves directionality or wave selectivity. Thus, if the coupler is so associated with a transmission line that an incident energy wave travels toward the right in main guide 5 as indicated by arrow ID, a corresponding secondary wave is excited toward the right in secondary guide 6 as indicated by arrow ll. Similarly, an oppositely directed reflected wave (not shown) in main guide 5, excites a secondary wave (-not shown) traveling toward the left in Fig.1.

Since it is generally desirable to utilize only coupled energy corresponding to an incident traveling wave l0, and to prevent interfering reflections, a conventional absorptive load or termination may be provided as shown in secondary guide 6 to absorb all coupled energy traveling in a backward direction relative to the incident Wave. A certain amount of energy may be coupled from the reflected wave in main guide 6 into secondary guide 6 in the direction of wave II, but this backward-1y coupled energy is so small compared to the coupled incident energy as to be negligible.

Referring now to Figures 2, 3, and 4 illustrating a preferred embodiment of the invention, and to sion line.

Fig. 2 in particular, main guide 5 is shown provided with flanges I6 and I! for coupling in a conventional manner to a waveguide transmis- Secondary guide 6 is terminated at one end by an absorptive load l5 designed to be effective over a relatively wide band of operating frequencies. The other end of the secondary guide 6 may be bent or curved, and also provided with a coupling flange I8, as shown, for connection to a utilization circuit. Main guide 5 and secondary guide 6 are interconnected through a metal block 20 which may be built up of sections, as shown in greater detail in Fig. 4. Channels are formed Within the block 20 to provide branch guides and 8 corresponding to those shown in Fig. 1.

Referring again to Fig. 2, the facing walls of main guide 5 and secondary guide 6 are suitably cut away to interfit closely with the block 20 and have intercommunication through the channels or branch guides l and 8 therein. The embodiment here shown thus corresponds to the basic structure of Fig. 1 described in detail.

It is to be understood that while a prefer-red embodiment of the invention has been here described, the invention is not limited to the physical construction illustrated in the drawing, and various changes and modifications may be made without departing from the spirit of the invention.

What is claimed is: g 4

1. A microwave energy coupler, including a main guide adapted to be associated with a transmission line carrying a traveling wave of electromagnetic energy, a secondaryguide, said main and secondary guides being rectangular in cross section and dimensionedto operate in a dominant mode, and a pair of spaced branch guides of rectangular cross section, said branch guides interconnecting said main and secondary guides and forming E-plane junctions therewith, the lengths and spacing of said branch guides being effectively odd multiples of a quarter guide- Wavelength at the operating frequency of said coupler, whereby directional coupling from the main guide to the secondary guide with minimum disturbance to wave energy in the main guide is secured.

2. A microwave energy coupler, including a main guide adapted to be associated with a transmission line carrying. a traveling wave of electromagnetic energy, a secondary guide, said main and secondary guides being rectangularin cross section and dimensioned to operate in a dominant mode, and a pair of spaced branch guides interconnecting said main and secondary guides and forming E-plane junctions therewith, the

lengths and spacing of said branch guides being effectively odd multiples of a quarter guidewavelength at the operating frequency of said coupler, whereby directional coupling from the main guide to the secondary guide with mini mum disturbance to wave energy in the main guide is secured.

3. A microwave energy coupler, including a main guide adapted to be associated with a trans mission line carrying a traveling wave of electromagnetic energy, a secondary guide, and a pair of spaced branch guides interconnecting said main and secondary Jguides, said main and 'sec-- of said coupler, whereby directionality in coupling action is secured.

4. A microwave energy coupler, including a main guide adapted to be associated with a transmission line carrying a traveling wave of electromagnetic energy, a secondary guide, and a pair of spaced branch guides interconnecting said main and secondary guides, said main and secondary guides being rectangular in cross section, the lengths and spacing of said branch guides being eifectively odd multiples of a quarter guide-wavelength at the operating frequency of said coupler, whereby directionality in coupling action is secured.

5. A directional coupler, including a main guide adapted to be associated with a transmission line carrying a traveling wave of electromagnetic energy, a secondary guide, and a pair of spaced branch guides interconnecting said main and secondary guides, the lengths and spacing of said branch guides being efiectively odd multiples of a quarter guide-wavelength at the operating frequency of said coupler.

EDWARD M. PURCELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

